Controlled atmosphere incubator

ABSTRACT

A controlled atmosphere incubator having an interior chamber surrounded by a heated water jacket. A glass access door of the chamber is directly heated by a clear, electrically conductive coating. The door is sealed against the perimeter of the opening by a readily replaceable gasket and is field reversible due to unique hinge mounting assemblies. An easily accessed blower assembly is located within the chamber and includes a HEPA filter readily replaceable by the user from within the chamber. A filtered air exchange system is provided for limiting the maximum level of humidity in the chamber. The incubator control maintains constant power output from the blower motor so that the heat output of the motor is also constant. A voltage compensated temperature control is also provided for the heaters associated with the water jacket. Compensation for environmental conditions inside the chamber is also provided by the control for producing more accurate readings of carbon dioxide levels. When an infrared carbon dioxide sensor is used, the control provides a unique calibration method which does not necessitate the use of conventional tanks of calibration gas mixtures.

This is application is a divisional of U.S. application Ser. No.09/615,270 filed Jul. 13, 2000, now abandoned which is a divisionalapplication of U.S. application Ser. No. 09/286,545 filed Apr. 5, 1999,now U.S. Pat. No. 6,117,687, which is a divisional of U.S. applicationSer. No. 09/110,574 filed Jul. 6, 1998, now abandoned, which is adivisional application of application Ser. No. 08/599,150 filed Feb. 9,1996, now U.S. Pat. No. 5,792,427.

BACKGROUND OF THE INVENTION

The present invention is generally related to controlled atmosphereincubators and, more specifically, to an improved incubator used toculture biological specimens.

Growing cell cultures in a laboratory incubator requires that theatmospheric conditions, such as temperature, humidity and gasconcentrations, remain constant throughout the incubator. A commonmanner of humidifying the culturing environment or incubator chamber isto place a stainless steel pan of water in the bottom of the incubator.The water in the pan evaporates and, since the incubator requires a gastight seal, the humidity level inside the incubator climbs to a levelabove 95% relative humidity. These high levels of humidity keep the cellcultures and their associated media from drying out during incubation.This is particularly critical when the volume of media is very small andthe time required to culture the cells spans, for example, several daysor more.

Although it is desirable to maintain these high levels of humidity forculturing and for fast humidity recovery after the incubator door isopened, it is not desirable to have condensate form anywhere inside theincubator. Condensate creates potential places for molds, spores andother unwanted bacteria to grow. Condensate will develop on any “coldspots” when the temperature on a surface is below the dew point of theair/gas mixture inside the incubator. Generally, incubators operate at atemperature of 37° C. and at elevated humidity conditions. The dewpoint, for example, at 37° C. and 98% relative humidity is 36.6° C.Therefore, any surface inside the incubator at a temperature below 36.6°C. and in contact with the air/gas mixture will condense the water fromthe mixture in the form of small droplets. These may then develop intopools or puddles of condensate. It is desirable for this reason as wellas others to maintain all interior surfaces at a constant temperature,however, there have been some practical limits that have required lessthan perfect conditions.

One location within the incubator where condensate tends to form is onthe inner glass door to the incubator chamber. Generally, these doorsare heated to prevent condensate from forming, especially after the doorhas been opened. For example, electric heaters are often placed in theoutside, insulated door and heat generated by these heaters isconducted, convected and radiated through the air space between theoutside, insulated door and the inner glass door. Because the heat mustbe transferred through the air gap between the two doors, heating of theinner glass door is relatively slow and inefficient. A more direct wayof heating the inner glass door is disclosed in U.S. Pat. No. 4,039,775.This patent discloses silk screened conductive elements or lines on theglass such as are commonly used in automobile window defrosters.Problems with such silk screened window defrosting elements, however,include reduced visibility through the glass door. If these lines areespecially close together, visibility is significantly reduced and ifthe lines are placed wider apart to increase visibility, sufficient heatmay not be transferred to the glass door. Also, these conductive lineelements tend to eliminate condensate only along the elementsthemselves, or if heated to the point that condensate is eliminated onthe entire glass panel, then too much heat may be generated and thechamber may be overheated. Finally, these conductive lines can bedamaged by abrasion and lose their conductive and heating capabilities.

Other problems associated with the inner glass door of laboratoryincubators involve the gasket which seals the door to the perimeter ofthe chamber opening and the mountings used to connect the glass door tothe incubator. Specifically, a gas tight seal is generally accomplishedusing a silicone “feather” gasket mounted around the opening of thechamber with the “feather” portion of the gasket providing a sealagainst the inner glass door in the closed position. To maintain theintegrity of the seal, the conventional method of mounting the gasket tothe chamber is by using a silicone adhesive/sealant. The gasket, alsogenerally formed of silicone, is extruded in a profile that creates agroove for the adhesive. These gaskets are difficult to clean because oftheir relatively complex geometry. A particularly dirty gasket may bereplaced in the field by peeling the gasket off the chamber, removingthe excess silicone adhesive and attaching a new gasket in the samemanner as the original one. This process, however, is tedious andrequires significant down time. With respect to the door mountings,hinges are generally permanently mounted to the chamber by spot welds.As these hinges may not be removed, the direction that the door swingsopen and closed is determined by the side of the chamber having thehinges. Field reversible doors have been an even more significantproblem in water jacketed incubators since these hinge mountings mustgenerally be placed through the water jacket portion of the incubator.

An air circulation system is also a vital ingredient in creating thecorrect environmental conditions for the growth of cell cultures in alaboratory incubator. Air circulation is needed to maintain temperatureuniformity within the chamber and also to effectively distribute and mixthe various gases, such as CO₂ and N₂, used to control the pH and O₂levels within the chamber. The air flow keeps the lighter gases fromstratifying within the chamber and aids in the control of CO₂ and O₂levels by providing air flow across the gas sensors. A blower isgenerally used in conjunction with a high efficiency particulate air or“HEPA” filter for circulating the air and removing contaminants from theair. The HEPA filters must be maintained at a temperature above the dewpoint of the air mixture to prevent condensation from developing insidethe filter. This condensation can restrict or block the flow of airthrough the filter. Problems which currently exist with such aircirculation systems include the requirement for an additional heatsource to maintain the temperature of the HEPA filter above the dewpoint of the air mixture. Also, HEPA filters have generally been mountedin locations requiring the removal of side panels and other hardwareassociated with the incubator in order to access the filter forreplacement. As the researcher or operator may be exposed to highvoltage components when removing these incubator panels, a qualifiedservice technician must be used for what should otherwise be a simplefilter replacement procedure.

While more complicated humidifying devices may be used to control therelative humidity within the chamber, the simplest device and mostcommon method involves placing a pan of water at the bottom of thechamber and allowing the chamber to become saturated through evaporationof water from the pan. Unfortunately, this simple method ofhumidification is not easily controlled and the resulting fullysaturated condition more easily leads to the development of condensationwithin the chamber.

With regard to temperature control within the chamber of the incubator,a variance in the line voltage applied to electric components whichgenerate heat will vary the heat output of the particular electriccomponent. Inconsistent heat output of such incubator components asheaters and motors makes it difficult to accurately and uniformlycontrol the temperature of the incubator chamber.

Laboratory incubators simulating biological conditions also generallyinclude carbon dioxide sensors to regulate the amount of CO₂ within thechamber and thereby simulate a specific pH or acidity level. Two generaltypes of CO₂ sensors are sensors based on a thermal conductivitydetection and sensors utilizing infrared technology. With respect tothermal conductivity CO₂ sensors, while these sensors are generally lesscostly, they are also sensitive to humidity and oxygen levels and totemperature variations within the chamber. While certain compensationsystems have been proposed, these systems have not entirely solved theproblems with environmental sensitivities. Infrared sensors are muchless sensitive to the above noted environmental conditions. However,calibration requires the use of a tank of gas having a specificpercentage of CO₂.

In view of the above noted problems and deficiencies of incubators ingeneral, there is a need for an incubator which provides a more accuratesimulated chamber condition and which is more easily operated andmaintained in the field by the end user.

SUMMARY OF THE INVENTION

The present invention is directed to an incubator which, in a firstaspect, includes a double door construction wherein one door comprisesan outside insulated door and the second door is an inside glass doorfor alternatively sealing and accessing the incubator chamber. Inaccordance with the present invention, the glass door is directly heatedby an electrically conductive, clear coating placed on at least onesurface of the door. The coating also has the characteristic of causingthe glass to have low emissivity.

Specifically, a dual glass pane configuration is used having two panesof glass separated by a space and with one pane being coated andelectrically heated. A pair of bus bars are disposed on opposite sidesof the glass door and provide for electric conduction across the coatedsurface. This glass door therefore provides direct and complete heatingof the glass to prevent or remove condensate on the inside surface ofthe glass without the disadvantages associated with indirect heatingmethods or typical silk screened conductive lines.

The glass door is sealed by a unique gasket which may be removed forcleaning and sterilization and replaced in a simple operation.Specifically, the gasket includes a mounting portion and an outwardlyextending feather portion which seals against the inside glass surface.The mounting portion simply presses onto the perimeter or edge of thechamber opening with a friction fit. The feather or sealing portion ofthe gasket extends outwardly in a manner and direction which preventsbuckling of the feather portion when mounted around the curved cornersof the door opening.

In accordance with a further aspect of the invention, the glass door isuniquely mounted onto the front water jacket portion of the incubator toallow the door to be easily reversed in the field from left to rightswinging or vice versa.

In accordance with another aspect of the invention, the air flow patternwithin the incubator is created by a blower assembly mounted within theincubator chamber in an easily accessible manner. Air is pulled into ablower near the top of the chamber and exhausted through duct work thatruns across the top of the chamber, down a plenum located behind a shelfsupport panel in the chamber and across the bottom of the chamber untilthe air disperses and is pulled up vertically through perforated shelveslocated inside the chamber. In accordance with the invention, a HEPAfilter is mounted directly to the blower and is located internally tothe chamber. Therefore, the blower assembly does not require anadditional heat source to maintain its temperature above the dew pointof the air mixture within the chamber. The HEPA filter is also easilyremoved and replaced by a researcher or other user from within thechamber and does not require the removal of side panels or otherhardware which might involve exposure to high voltage wiring and/orcomponents.

The invention further contemplates a control volume approach toregulating the humidity level within the chamber. Specifically,relatively low humidity ambient air is drawn into the inlet of theblower through a filter and, at the same time, air exits the chamberthrough a filtered outlet. This air exchange prevents completesaturation of the incubator chamber and therefore assists in preventingthe formation of condensation inside the incubator. HEPA filters arepreferably used to filter both the incoming ambient air as well as theair exiting the chamber. This minimizes the potential for introducingcontamination into the incubator and allowing escape of contaminationfrom the incubator. As this air exchange system is formed as part of theinternal air circulation system in this unique manner, no additionalcomponents or electronics are necessary for controlling the relativehumidity within the chamber.

The motor for operating the blower is mounted directly above and againstthe top of the incubator chamber. The control of the present inventionoperates to maintain constant power, and therefore constant heatgeneration, from the motor during voltage and frequency variations.Specifically, the invention features a method and apparatus forcontrolling application of a line voltage to an electric motor in thechamber, to reduce variations in heat produced by the motor that mightbe caused by line voltage variations. In accordance with this aspect,the line voltage and/or frequency is measured and, based on performancecharacteristics of the motor, the line voltage is applied to the motorin a pattern which will result in a predetermined net root-mean-squarevoltage across the motor, and consequently a predetermined amount ofheat.

In particular embodiments, the line voltage is an AC line voltage,applied to the motor through a triac. The triac is activated a variabledelay time after a zero crossing of the line voltage (and automaticallydeactivates when the motor current reaches zero). The appropriate delaytime for a given RMS line voltage is determined by applying various linevoltages and frequencies, and various delay times, to determine a delaytime for each line voltage and frequency which results in apredetermined RMS voltage across the motor's terminals. This delay timeis then stored in the table for later retrieval and use in controllingthe motor.

In accordance with another aspect, the operation of a heater under thecontrol of a closed-loop temperature control system is improved byreducing variation in the heater's heat output caused by line voltagevariation. The closed-loop temperature control is calibrated foroperation at a predetermined line voltage, such as 90 Volts RMS, and inoperation generates a heater power fraction indicating the fraction offull heater power to be applied to the chamber. The control circuitdetermines a ratio of the root-mean-square amplitude of the measuredline voltage to (90)². Then, a revised heater power fraction is obtainedby dividing the heater power fraction demanded by the closed-looptemperature control system by the computed ratio. The heater thengenerates the revised heater power fraction of its maximum heat output.As a result, the closed-loop temperature control system will obtain aconsistent heat output from the heater independent of variations in linevoltage.

In preferred embodiments, the line voltage is an AC line voltage whichis alternately applied, and not applied, to the heater, such that poweris applied to the heater for a fraction of time equal to the revisedheater power fraction.

In accordance with another aspect, a gas sensor such as a thermalconductivity carbon dioxide sensor is calibrated to compensate forsensitivities to oxygen and/or humidity, by measuring the temperatureand humidity and/or oxygen in the vicinity of the gas sensor, and thendetermining a humidity and/or oxygen variation between said the currenthumidity/oxygen level and the humidity/oxygen level extant when the gassensor was calibrated. Next, the incubator temperature is used todetermine an amount of humidity and/or oxygen variation which wouldcause a one percent apparent change in gas concentration indicated bythe gas sensor. Then the humidity and/or oxygen variation is divided bythe amount of humidity/oxygen variation which would cause a one percentapparent change in gas concentration, and the resulting value is addedto the gas concentration indicated by the gas sensor, thus compensatingthe gas sensor reading for variations in humidity and/or oxygen.

In preferred embodiments, the humidity sensor measures a relativehumidity level, and this relative humidity level is converted to grainsbased on the measured temperature, so that the grains may be used tocompute the apparent change in gas concentration caused by humidityvariation.

In another aspect, the invention features a circuit for detectingwhether an incubator door is open. The door has a conductive frameattached thereto which is electrically isolated from the grounded frameof the chamber. When the door is closed, the conductive door framecontacts the chamber frame and the door frame is thereby grounded.However, when the door is open, no such contact is made, so that apull-up circuit electrically connected to the door frame pulls the doorframe up to a logic “high” voltage (e.g., 5 volts) which can be detectedby logic circuitry in the controller and used to indicate that the dooris open.

In preferred embodiments, the logic circuitry includes electrostaticisolation circuitry which protects the controller from electrostaticdischarge in the door conductor.

An optional infrared based carbon dioxide sensing element or detector isalso contemplated by the present invention. In accordance with theinvention, a unique calibration method eliminates the need for aseparate supply of calibrating gas. This calibration procedure takesadvantage of uniform ambient air conditions wherein CO₂ represents0.033% of the volume of ambient air. Specifically, a predeterminedconcentration of gas is applied to the sensor, and then the sensoroutput is measured when the normal power is applied to the light source,and when a reduced power level is applied to the light source. These twomeasurement points are then curve-matched to a known, predeterminedcurve of source power vs. sensor output for the sensor, to produce anoffset and gain factor to be applied to further sensor readings.

Further objects and advantages of the present invention will become morereadily apparent to those of ordinary skill upon review of the followingdetailed description taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an incubator constructed in accordancewith the present invention;

FIG. 2 is a fragmented cross sectional view of the inner glass door inperspective and taken generally along line 2—2 of FIG. 1;

FIG. 2A is a cross sectional view of the gasket shown in FIG. 2 butremoved from the incubator;

FIG. 3 is a front elevational view of a fastener receiving element usedto mount the inner glass door to the incubator cabinet;

FIG. 4 is a cross sectional view of the fastener receiving element takenalong line 4—4 of FIG. 3;

FIG. 5 is a diagrammatic front view showing the air flow pattern andblower assembly within the incubator chamber;

FIG. 6 is a fragmented perspective view showing the blower assembly andCO₂ sensor at the top of the incubator chamber with an upperplenum-defining plate removed for clarity;

FIG. 7 is a perspective view of the incubator cabinet with the upperdrawer opened to show various control components therein;

FIG. 8 is a perspective view of the interior top of the incubatorcabinet which holds the motor for operating the blower assembly;

FIG. 9 is a block diagram of the control and sensor circuitry used forthe chamber's environmental control system;

FIG. 10A is a circuit diagram of the RMS to DC Converter circuit of FIG.9;

FIG. 10B is a circuit diagram of the Zero Crossing Detector of FIG. 9;

FIG. 10C is a circuit diagram of the Fan Motor Control of FIG. 9;

FIG. 11A is a flow chart of operations taken by the microprocessor ofFIG. 9 to compensate the motor power consumption for line voltagevariations;

FIG. 11B is a timing diagram useful in understanding the circuit of FIG.10C and operations described in FIG. 11A;

FIG. 12 is a flow chart of operations taken by the microprocessor ofFIG. 9 to compensate the heater power consumption for line voltagevariations;

FIG. 13A is a flow chart of operations taken by the microprocessor ofFIG. 9 to compensate readings of the CO₂ Sensor of FIG. 9 for variationsin humidity;

FIG. 13B is a mathematical operation diagram illustrating the method ofcompensation described in FIG. 13A;

FIG. 14A is a flow chart of operations taken by the microprocessor ofFIG. 9 to compensate readings of the CO₂ Sensor of FIG. 9 for variationsin oxygen;

FIG. 14B is a mathematical operation diagram illustrating the method ofcompensation described in FIG. 14A;

FIG. 15A is a flow chart of operations taken by the microprocessor ofFIG. 9 to calibrate an infrared CO₂ sensor;

FIG. 15B illustrates stored curves used by the microprocessor of FIG. 9to calibrate an infrared CO₂ sensor; and

FIG. 15C is a block diagram of an infrared CO₂ sensor and the circuitryused by a microprocessor 300 to calibrate this sensor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates an incubator 10 constructed in accordance with thepresent invention and generally including an insulated, and preferablywater-jacketed, cabinet 12 with an interior controlled atmospherechamber 14. Chamber 14 is accessed through a pair of doors which includean outer insulated door 16 and an inner heated glass door 18. Insulateddoor 16 is attached to cabinet 12 by a pair of hinges 20, 22 which maybe alternatively attached to the left or right side of cabinet 12depending on which direction it is desired to swing insulated door 16.Likewise, inner glass door 18 includes hinges 24, 26 secured byfasteners 28 to front panel 40 of cabinet 12. Fasteners 28 and theirassociated receiving elements will be described in more detail below,however, in general these fasteners 28 and receiving elements 30 allowfastening of door 18 to front panel 40 in either a left or rightswinging manner. Fastener receiving elements 30 are installedpermanently on both the left and the right side of front panel 40 andare sealed into the water jacket portion of cabinet 12. Door 18 furtherincludes a latch assembly 32 having a twist latch 34 fastened to frontpanel 40 by fasteners 36. Latch 34 bears against frame 38 of door 18when in the latched position to seal door 18 against front panel 40 aswill be described below.

FIG. 2 illustrates the specific construction of inner glass door 18 aswell as the manner of forming a gas-tight seal between door 18 and frontpanel 40 of cabinet 12. Inner glass door 18 includes two glass panes 42,44 separated by an aluminum spacer 46 to create an intermediate air gap48. Inner pane 44 includes a low emissivity coating 50 on the innersurface thereof between a pair of bus bars 52, 54 silk screened overedge portions of coating 50. Coating 50 preferably comprises aconventional clear, tin oxide coating. Bus bars 52, 54 are electricallyconnected to a control, to be described below, to apply currenttherebetween and through coating 50. A channel 55 is created withinframe 38 on the hinge side of glass door 18 to route wiring from busbars 52, 54 to the control to be described. Coating 50 is heated andprevents the formation of condensation. If condensation has alreadyformed on the chamber side of pane 44, such condensation is evaporatedby the heat generated by coating 50. The entire perimeter 60 of innerpane 44 is void of coating 50 so that conduction does not take placethrough the aluminum spacer 46 in a manner which bypasses coating 50.

FIG. 2 also illustrates the unique gasket 64 of the present invention asit seals against inner pane 44. Gasket 64 specifically includes amounting portion 66 which is trough-shaped or U-shaped in cross sectionso that it may easily fit over the edge 40 a of front panel 40 whichdefines the opening to chamber 14 (FIG. 1). A sealing portion or feather68 extends outwardly from mounting portion 66 and maintains sealingengagement with inner glass pane 44 when door 18 is in the closedposition. Mounting portion 66 and feather 68 are preferably extrudedtogether from silicone and, extruded within mounting in portion 66 is awire 70 which lends structural rigidity to gasket 64. The U-shapedmounting portion 66 further includes locking elements 72 which engage onopposite sides of panel 40 to retain gasket 64 in position about theopening to chamber 14.

Referring now to FIG. 2A, gasket 64 is uniquely constructed such thatfeather 68 will not buckle or become rippled when gasket 64 is bentaround the relatively small radius corners of the opening to chamber 14(FIG. 1). The angle α and the placement of feather 68 with respect tothe approximate bending plane 74 of gasket 64 are chosen such that tip68 a of feather 68 is positioned inside of the approximate bending plane74. That is, tip or edge portion 68a lies on the side of bending plane74 which is closer to the open end 76 of the U-shaped mounting portion66. In the preferred embodiment, angle α is approximately 80°.

Referring now to FIGS. 3 and 4, a fastener receiving element 30 forfastening door 16 to cabinet 12 is shown in greater detail. As shownbest in FIG. 4, these fastener receiving elements 30 are fastened ontofront panel 40 and extend through an opening 77 in front panel 40 into awater jacket 78. Water jacket 78 is defined between front panel 40 and asecond inner panel 80. Water jacket 78 preferably extends along the top,bottom, rear and two sides of cabinet 12. As also shown in FIG. 4,opening 77 in front panel 40 is deformed such that a stepped downportion 82 is formed for receiving a head or flange 84 of the fastenerreceiving element 30. As shown in FIG. 3, both the stepped down portion82 and the head or flange 84 of receiving element 30 are formed with thesame, noncircular shape such that head 84 may not rotate with respect tofront panel 40 when a fastener 28 (FIG. 1) is threaded into receivingelement 30. In the preferred embodiment, both the stepped down portion82 and head 84 have a “race track” shape with two parallel curved sides82 a, 84 a and two parallel straight sides 82 b, 84 b.

Referring back to FIG. 4, opening 77 in panel 40 which accepts receivingelement 30 further includes an angled portion 86 which acts as a sealingsurface with an O-ring 88 situated between flange or head 84 and angledportion 86. Finally, fastener receiving element 30 includes internalthreads 90 which accept both a tool for crimping the receiving element30 into place on front panel 40 and accepting conventional threadedfasteners 28 for removably securing glass door 18 to front panel 40(FIG. 1). With regard to the crimping process for attaching receivingelement 30 to front panel 40, it will be appreciated from FIG. 4 that atool, such as a pneumatic tool, is used to pull the inner threadedportion 92 of element 30 toward head or flange portion 84 to create anoutwardly deformed area 94 which bears against angled portion 86. Thistightly traps O-ring 88 between flange or head portion 84 and angleportion 86. In this way, a water tight seal is created which preventsleakage from water jacket 78 at the site of each fastener receivingelement 30.

Referring now briefly to FIG. 1, it will be appreciated that theplacement of fastener receiving elements 30 in upper and lower positionson both the right and left hand sides of front panel 40 allows glassdoor 18 to be easily switched from left to right swinging by simplyremoving fasteners 28 from, for example, the left side, flipping door 18over and fastening hinges 24 and 26 to the right side fastener receivingelements 30 using the same threaded fasteners 28. As also shown in FIG.1, the side of door 18 which includes hinges 24, 26 also includesidentical electrical connectors 96, 98 on upper and lower edges thereoffor making the appropriate electrical connection to bus bars 52, 54(FIG. 2) at an upper end of door 18 whether door 18 is swinging from theleft or the right. Both electrical connectors 96, 98 are connected towiring (not shown) contained in channel 55 (FIG. 2). This makes theelectrical connection with electrical control hardware components to bedescribed below easier as these components are contained in the top ofincubator 10.

Referring now to FIGS. 5 and 6, the unique HEPA air circulation systemof the invention includes a blower assembly 100 which is mounted to aplate 102 (FIG. 6) at an upper end of chamber 14. Mounting plate 102 isfastened to an upper panel 104 of chamber 14. As appreciated from FIG.5, an outlet 106 of blower assembly 100 is situated between panel 104and a panel 108. An upper plenum 110 is defined between panels 104, 108and extends across the top of chamber 14. In FIG. 6, panel 108 has beenremoved for clarity but, in practice, sits against surface 112 of blowerassembly 100 between outlet 106 and an inlet 114 to which is attached aHEPA filter 116. HEPA filter 116 is mounted to a cylindrical extension118 defining inlet 114 of blower assembly 100. Extension 118 includes anO-ring seal 120 for sealing the removable connection made between HEPAfilter 116 and extension 118. As appreciated from FIG. 5, HEPA filter116 may be easily removed and replaced from within chamber 14. Plate 108is removably fastened in the upper portion of chamber 14 to a pair ofthreaded elements 122, 124 extending downwardly from mounting plate 102.Thus, plate 108 may also be easily removed to allow maintenance andreplacement of the entire blower assembly 100 from within chamber 14.Also extending from plate 102 is a temperature probe 126 and a humiditysensor 128. Probe 126 and humidity sensor 128 are connected to furthercontrol hardware located at the top of incubator 10 and function in amanner to be described. Also contained within upper plenum 110 is a CO₂sensor 130 which may be of a thermal conductivity or infrared variety aswill also be described below. The CO₂ sensor 130 is therefore mounted inthe path of filtered air exiting blower assembly 100 and may also beeasily accessed from within chamber 14 after removing plate 108. A HEPAfiltered sample port 131 is also mounted to plate 102 for drawing testsamples of air from chamber 14.

As shown in FIG. 5, chamber 14 further includes a side plenum 132 whichconnects with upper plenum 110 and which has a lower opening 134. Sideplenum 132 is disposed behind a shelf support panel 133 within chamber14. Air is drawn into the HEPA filtered inlet 114 (FIG. 6) of blowerassembly 100, exits across upper plenum 110 and past CO₂ sensor 130. Thefiltered air then moves downwardly through side plenum 132, throughopening 134 and across a conventional pan 136 which holds water forhumidifying chamber 14. As shown in FIG. 1, shelves 138 mounted withinchamber 14 are perforated to allow air circulation upwardly and finallyback through HEPA filter 116.

Referring now to FIG. 7, the upper end of cabinet 12 includes a drawer140 for conveniently holding the various hardware components 142, 144,146, 148 as well as others, not shown, which are necessary in theimplementation of the control to be described below. Drawer 140therefore allows easy maintenance and replacement of these componentswithout necessitating the removal of panels as is conventional. Asfurther shown in FIG. 7, an air inlet line 150 extends into drawer 140and includes a HEPA filter 152 attached on the end. This air inlet line150 is used to draw in relatively dry or low humidity ambient air intochamber 14 by being connected to the inlet of blower assembly 100. Asshown in FIG. 8, line 150 extends through mounting plate 102 and issuitably attached to communicate with the inlet of blower assembly 100(FIG. 5). Filter 152 controls the amount of ambient air being drawn inand is preferably a HEPA filter which has a flow rate of 1/10 c.f.m.Other manners of regulating the flow rate to 1/10 c.f.m. are alsopossible. As shown in FIG. 5, a HEPA-filtered outlet 154 is disposed inthe back wall of chamber 14 and also leads to the ambient environment.The same HEPA filter as filter 152 is preferably disposed within outlet154. The constant exchange of air, i.e., by ambient air being drawn inthrough inlet 150 at about 1/10 c.f.m. and chamber air leaving throughoutlet 154 at the same rate, provides a simple control volume approachto help insure that chamber 14 does not reach a saturated state. Thishelps prevent the formation of condensation within chamber 14.

Referring back to FIG. 8, a motor 156 is secured to mounting plate 102and extends through an opening 158 in upper panel 104. A gasket 160 isdisposed between mounting plate 102 and upper panel 104. Thisarrangement allows motor 156 to be mounted outside of incubator 14 butdirectly adjacent thereto so that it may be easily connected to blowerassembly 100 (see FIGS. 5 and 6). A top section 162 of water jacket 78is formed with a portion 164 cut out to allow the mounting of motor 156directly against mounting plate 102 and to also allow the filteredambient air inlet 150 as well as other control components to extend intoincubator 14 from, for example, drawer 140 (FIG. 7). As also shown inFIG. 8, water jacket 78 is preferably thermally connected with electricheating elements 79 for directly heating the water therein andindirectly heating incubator 14 in a uniform manner (FIG. 1). Althoughelements 79 are shown in the top of water jacket 78, it will beunderstood that such elements may also be placed, or alternatively beattached outside water jeacket 78 in the same or other locations.

Referring now to FIG. 9, the electrical control system for theincubator's environmental control is centered around a microprocessor300 which receives data from sensors and a number of other sources, andcontrols the heater, fan and gas flow into the incubator.

Microprocessor 300 produces displays on a character display on the frontof the unit, and is able to receive commands from a user throughkeystrokes on keys located adjacent to the display. This display and thekeys next to it form a user interface 302 through which microprocessor300 can be controlled by a user. Using this user interface, the user mayset the incubator temperature, set the carbon dioxide or other gaslevels in the incubator, and enable calibration of the various sensorsused by the incubator to control these environmental parameters.

As is discussed in further detail below, the performance of the heaterand of the fan motor are subject to variation upon variation of the ACline voltage or AC line frequency produced by the wall outlet from whichthe circuitry is drawing power. To compensate for these possiblevariations, the voltage and the frequency of the AC power are detected,and the voltage is used to compensate the heater, while voltage andfrequency are used to compensate performance of the fan motor. The ACline voltage is represented in FIG. 9 by voltage source 304. A zerocrossing detector 306 is used to detect the frequency of the AC linevoltage from source 304. At the same time, a RMS to DC converter 308 isused to detect the amplitude of the AC line voltage. Zero crossingdetector 306 produces a logic signal pulse on line 307 corresponding toeach zero crossing of the AC line voltage 304. These logic pulses areused by microprocessor 300 to determine the frequency of the AC linevoltage. RMS to DC converter 308 determines the amplitude of the AC linevoltage and produces an analog output voltage on line 309 proportionalthereto. This analog output voltage is converted by an A/D converter 310to a digital value which may be used by microprocessor 300. As iselaborated in further detail below, the voltage amplitude of the AC linecan then be used by microprocessor 300 to calibrate the operation ofincubator heaters and used in conjunction with the frequency to controlthe fan motor.

Microprocessor 300, as noted above, controls the temperature and carbondioxide level within the incubator to desired levels requested by theuser through user interface 302. To perform this control, microprocessor300 obtains a reading of the carbon dioxide level in the incubator froma carbon dioxide sensor 312, and obtains a reading on the temperature inthe incubator from a temperature sensor 314.

Temperature sensor 314 is a temperature sensitive electrical device suchas a thermistor coupled to analog circuitry which produces an analogvoltage on line 315 proportional to the temperature in the incubator.This analog voltage is converted by A/D converter 310 to a digital valuewhich may be read by microprocessor 300.

Carbon dioxide sensor 312 may be a thermal conductivity carbon dioxidesensor or an infrared carbon dioxide sensor. Thermal conductivity carbondioxide sensors measure thermal conductance between two points and fromthis produce an analog signal which is representative of the carbondioxide content in the region between those two points. This kind ofcarbon dioxide sensor is relatively inexpensive, however, it issensitive to humidity and oxygen level variation and thus is susceptibleto errors if such variations occur in the incubator. Infrared carbondioxide sensors are not sensitive to humidity and oxygen levels,however, such sensors are expensive (due to various complex supportingcircuitry) and may be complex to calibrate using known procedures. Eachof these drawbacks of carbon dioxide sensors is alleviated throughprinciples of the present invention, as discussed below.

When a thermal conductivity carbon dioxide sensor is used for sensor312, in accordance with principles of the present invention,sensitivities to humidity and oxygen concentration are compensatedthrough the use of humidity and oxygen sensors 316 and 318.

Humidity sensor 316 is a relative humidity sensor which produces asignal related to relative humidity within the incubator. This signal isimpressed on line 317 leading to A/D converter 310, and is converted toa digital signal read by microprocessor 300 to determine the relativehumidity within the incubator.

Oxygen sensor 318 produces a millivolt level signal linearly related tothe oxygen concentration within the incubator. This analog signalappears on line 319 and is converted by A/D converter 310 for use bymicroprocessor 300. The output signal from oxygen sensor 318 isconverted to a signal representative of oxygen content by applying ahigh oxygen level to oxygen sensor 318, and storing the sensor outputlevel for this extreme of oxygen concentration. The sensor output at 0%oxygen is approximately 0 volts. Further oxygen signal levels may bemeasured against those extremes using linear interpolation to determinea corresponding oxygen level. An operation of this kind is discussedbelow in FIG. 13A and 13B in connection with linearization of readingsfrom carbon dioxide sensor 312.

A final input signal to microprocessor 300 is obtained from a door opendetection circuit on line 321. The frame of the incubator door 320 is aconductor of a switch circuit, and is isolated from the frame of theincubator whenever the door is open. When the door is closed, door frame320 electrically contacts the incubator and thereby becomes groundedthrough a connection 323. A pullup voltage (for example, 5 volts) isapplied through a pullup resister 322 to cause door frame 320 to elevateto the pullup voltage whenever the door is open and not contacting theincubator frame. However, when door frame 320 is contacting theincubator frame, door frame 320 is at a ground potential. Thus, thevoltage of door frame 320 is a logic-level signal indicative of whetherdoor frame 320 is in contact with the incubator frame or not, and thusindicates whether the door is open. This signal is fed directly to themicroprocessor 300 on line 321. Circuit 324 includes electrostaticdischarge protection circuitry coupled to line 321 to protectmicroprocessor 300 and other electrostatic sensitive circuitry in theincubator controller from possible electrostatic discharge which mayoccur into door frame 320.

Microprocessor 300 evaluates and processes all of the signals describedabove from various input sources in accordance with procedures discussedin the following figures. In these procedures, microprocessor 300 makesuse of a number of lookup tables found in a memory 330. These tablesinclude motor control lookup tables 331 and 332. Table 331 is used forcontrolling fan motor when 60 Hz AC power is being applied, and table332 is used to control the fan motor when 50 Hz AC power is beingapplied.

Also stored in memory 330 are conversion and linearization lookup tables334 which are used to convert and/or linearize signals obtained fromvarious climate control sensors discussed above. Specifically, forexample, temperature sensor 314 produces an analog signal which isroughly linear with temperature, but includes some variations from alinear curve. Microprocessor 300 converts temperature readings obtainedthrough A/D converter 310 by using the temperature reading obtainedthrough A/D converter 310 as an address into a lookup table 334, toretrieve a corresponding compensated linearized temperature reading. Thelookup table 334 used for temperature sensor linearization is generatedfrom published formulas for thermistor characteristics obtained from themanufacturer of the thermistor used as temperature sensor 314, and isable to compensate temperature sensor readings to an accuracy from about±0.2° C.

Another lookup table 334 is used to convert digitized signals fromhumidity sensor 316 into corresponding relative humidity values. Asnoted above, humidity sensor 316 produces an analog signal related torelative humidity. Once digitized, this relative humidity signal isconverted to a digital number indicated 0-100% relative humidity using asecond lookup table 334,

A third lookup table 334 is used to linearize signals from carbondioxide sensor 312. The infrared carbon dioxide sensor output signal isnot linear with variations in carbon dioxide concentration. The lookuptable compensates for these nonlinearities. The thermal conductivity CO₂sensor output is linear with variations in CO₂ concentration.

The last set of tables 336 stored in memory 330 is used, in a mannerelaborated below, to calibrate the output of a thermal conductivitycarbon dioxide sensor 312, by compensating for sensitivities of thissensor to humidity variations. The details of the use of these tableswill be elaborated below.

Microprocessor 300 is also connected to various climate control elementsin the incubator. These include a main heater 340 which is connected tothe AC line voltage to produce heat to warm a water jacket surroundingthe incubator, and secondary heaters 342 which are used to heat theinner door and front of the chamber. In addition, a fan control 344 isused to control a fan motor 346 to circulate air within the incubator. Agas flow control circuit 348 is used to control a valve 350 to permitgas (such as carbon dioxide) from a gas source 352 to enter incubator354. Finally, if an infrared carbon dioxide sensor 312 is in use,microprocessor 300 controls a source 356 of infrared light used tostimulate the infrared carbon dioxide sensor 312. (See FIG. 15C, below.)

Referring now to FIG. 10A, the RMS to DC converter 308 is an analogcircuit for detecting the amplitude of the voltage of the AC power line304. The circuit includes a transformer 360 for stepping the AC linevoltage down to a manageable level, followed by a filtering circuit 362comprising two resistors and a capacitor, which removes high frequencynoise from the AC line 304. The output of this filter feeds an RMS to DCchip 364 which produces an output on line 365 which is an analog voltagerepresentative of the root mean square amplitude of the voltage on ACline 304. A suitable RMS to DC chip 364 can be purchased from AnalogDevices of Norwood, Mass. as Part No. AD736. The analog voltage on line365 is conditioned by an operational amplifier circuit 366 configured asa follower circuit. The resulting conditioned analog signal on line 309is converted by A/D converter 310 to a digital signal indicative of theAC line amplitude.

Referring now to FIG. 10B, the zero crossing detector 306 is an analogcircuit connected to AC line 304, which detects reversals in polarity ofAC line 304. In this circuit AC line 304 is connected through resisters370 to a pair of light emitting diodes, and an associated opticallycoupled transistor. The diodes and optically coupled transistor areenclosed within a single optical isolation chip 372. Whenever AC line304 is at a substantial positive or negative voltage, one of the diodesin chip 379 is turned “on”, and as a result the transistor in chip 372is turned on, drawing node 373 near to a ground potential. However, whenAC line 304 nears zero, current does not flow through either of thelight emitting diodes in chip 372, and these diodes turn off. Thiscauses the transistor in chip 372 to turn off, after which node 373raises to a pullup voltage. Thus, a positive pulse is produced on node373 during each zerocrossing of the AC line. An invertor 374 isconnected to node 373, so that circuit 306 produces a signal on line 307that has a logic “0” pulse occurring at each zero crossing of the ACline, and otherwise has a logic “1” value. (See FIG. 11B.)

Now referring to FIG. 10C, the fan motor control circuit 344 is asimilar analog circuit driven by a motor control signal. This motorcontrol signal drives the base of a transistor 376, the collector ofwhich is connected to an optical isolation chip 378. Optical isolationchip 378 optically couples a diode to a diac 379, such that when currentflows through transistor 376, diac 379 is turned “on” and creates ashort circuit between its terminals. Diac 379 is, in turn, coupledacross the control and one signal terminal of a triac 380. Triac 380 isconnected between the AC line 304 and the fan motor 346.

When diac 379 is activated, it turns triac 380 “on”, causing the AC linevoltage 304 to be applied across motor 346 and generating current flowthrough the motor 346. Current will flow through motor 346 in responseto AC line voltage 304 until some time later, when AC line 304 reversespolarity, and the current in motor 346 reduces to zero. When the currentin motor 346 reduces to a zero value, triac 380 ceases to conduct, andwill remain nonconducting until reactivated by triac 379 in response toa motor control signal applied to transistor 376.

Thus, a motor control signal applied to transistor 376 will cause the ACline voltage 304 to be applied to motor 346 for approximately one-halfcycle of the AC line 304. Further application of AC line voltage 304 tomotor 346 will be delayed until a motor control signal to transistor 376reactivates triac 380. The delay time between the end of a half cycle ofthe AC line 304, and the time of reactivation of triac 380, is used asdiscussed below to control the total power applied to motor 346.

Specifically, referring now to FIG. 11B, the AC line voltage 304 has aroughly sinusoidal waveform including periodic zero crossings at timesrepresented by lines 390. Through operation of the circuitry describedabove in FIG. 10B, the zero crossing signal includes a brief pulse to a“0” logic level at each of the zero crossings, and otherwise has a “1”logic level. Microprocessor 300 responds to this zero crossing signal byproducing a motor control signal to transistor 376 a time delay aftereach zero crossing. This time delay, represented by the gap 392 betweena zero crossing line 390 and a motor control signal pulse, is varied bymicroprocessor 300 in a manner to control the total power applied to fanmotor 346 to a constant predetermined value.

Specifically, referring to FIG. 11A, microprocessor 300 receives zerocrossing signals on line 307, and computes a time delay between thesezero crossing signals to determine whether the AC line frequency is 50Hz or 60 Hz (step 400). Next, microprocessor 300 determines the RMSamplitude of the AC line voltage from the RMS to DC converter 308. Thisamplitude is used to select an entry in either the 60 Hz lookup table331 or 50 Hz lookup table 332, depending on whether the AC linefrequency is 60 Hz or 50 Hz (step 402). The selected entry in the lookuptable 331 or 332 identifies the time delay 392 between each zerocrossing and each motor control signal. This off delay is read (step404) from the lookup table, and then used by microprocessor 300 inproducing further motor control signals.

When the fan in the climate controlled incubator is in operation, asubstantial amount of electrical energy is consumed, as heat, in thecoils of the fan motor. This heat energy affects the climate control ofthe incubator by acting as a second heater within the incubator. Whilethis source of heat cannot be eliminated, it can be controlled so thatthe amount of heat produced by fan motor 346 is constant despitevariations in line frequency and line voltage. Both the line frequencyand the line voltage will affect the response of the fan motor 346 tothe AC line voltage through the operation of circuit 344 of FIG. 10C.Specifically, fan motor 346 has a highly reactive electrical impedance,and will produce a motor current which depends upon the frequencyapplied to the motor, as well as the amplitude. Furthermore, the currentdrawn by fan motor 346 is related to the amplitude of the AC linevoltage in a non-linear fashion.

To compensate for these various effects caused by the non-linear andreactive impedances of fan motor 346, lookup tables 331 and 332 aregenerated and filled with values for time delay 392. The values storedin the tables 331 and 332 indicate the delay time which, when used withthe associated AC line frequency and amplitude, will cause fan motor 346to produce a predetermined heat output. Lookup tables 331 and 332 aregenerated by applying various line frequencies and line amplitudes to afan motor 346, and using the circuit of FIG. 10C to vary the motorcontrol signal pulse time delay 392 while monitoring the RMS voltageappearing across the terminals of the fan motor 346. For eachcombination of line frequency and amplitude, the time delay 392 isvaried until a predetermined desired RMS voltage appears across theterminals of fan motor 346. The time delay which achieves thispredetermined RMS voltage is stored in lookup table 331 or 332 in theappropriate location. By filling out lookout tables 331 and 332 in thismanner, when microprocessor 300 later retrieves a value from a lookuptable 331 or 332, this value will be the appropriate value to controlfan motor 346 to generate the desired predetermined amount of heatenergy regardless of variations in the AC line frequency and amplitude.

Referring now to FIG. 12, a similar compensation is performed in thecontrol of heaters 340 and 342. Heaters 340 and 342 are controlled bymicroprocessor 300 to produce a desired heat output. Specifically,microprocessor 300 is programmed to implement an adaptiveproportional-integral-derivative control algorithm, responsive totemperature readings from temperature sensor 314, to control thetemperature inside the incubator. A particular suitable algorithm is theISA “ideal algorithm for closed loop PID control”. This controlalgorithm generates a value indicative of the heater power that shouldbe applied at any given time to properly control the temperature of theincubator. This heater power is represented by a percentage of maximumheat. Microprocessor 300 controls main heater 340 or secondary heaters342 to produce the desired percentage of maximum heat, by alternatelyapplying the AC line voltage 304 to the heater coil, such that the dutycycle with which the AC voltage is applied equals the desired percentageof maximum heat.

The heat produced by the heater 340 or 342 is a function not only of theduty cycle created by microprocessor 300, but also of the RMS AC linevoltage which is applied to the heater. Although the adaptive controlalgorithm implemented by microprocessor 300 could over time compensatefor different AC line voltages (by self-adjustment of its adaptiveparameters), this adaptation could take hours or even days, during whichthe temperature control of the incubator will be unacceptably out ofcontrol. To avoid the need for dynamic adaptation of the controlalgorithm in microprocessor 300, microprocessor 300 more directlycompensates for AC line voltage, to eliminate the potential effects ofvariation in the RMS AC line voltage applied to the heater 340 or 342.

Specifically, the adaptive control algorithm used by microprocessor 300is initialized using a 90 volt AC RMS line voltage. Then, the actual ACline voltage is measured and incorporated into the control for heater340 or 342 to compensate for variations of the AC line voltage away fromthe nominal 90 volts RMS line voltage used in initializing the adaptivecontrol algorithm.

In this compensation process, microprocessor 300 begins by reading theRMS amplitude of the AC line from circuit 308 via A/D converter 310(step 410). Next, microprocessor 300 computes the ratio of 90² to thesquare of the measured RMS voltage (step 402). This ratio is a heatreduction factor indicative of the reduction in duty cycle needed tocompensate for additional heat energy that will be produced by theheater due to additional RMS AC line voltage above the nominal 90 voltlevel used in calibration of the adaptive control algorithm.Accordingly, in step 414, microprocessor 300 multiplies this gain factorby the percent power requested by the adaptive control algorithm toproduce the actual heater duty cycle (on-time) percentage to be used ingenerating heat from heater 340 or 342. In this way, any additionalheater power which might have been generated by AC line RMS voltagegreater than 90 volts, is compensated by reduction in the on-time of theheater 340 or 342.

Referring now to FIG. 13A, as discussed above, a thermal conductivitycarbon dioxide sensor 312 is compensated for variations caused byhumidity and linearized by microprocessor 300. This process isillustrated in FIG. 13A and charted in FIG. 13B.

To initiate this process, the carbon dioxide sensor outputs are measuredat low and high levels of CO₂ and used to interpolate further signalvalues. To do so, the carbon dioxide level in the incubator is reducedto a low level (for example by flooding the incubator with ambient roomair) (step 420), and then microprocessor 300 measures and stores thecarbon dioxide sensor reading at this low carbon dioxide level. At thesame time, microprocessor 300 obtains and stores the actual carbondioxide level (e.g., from operator input) (step 422). Next, the carbondioxide level in the incubator is increased to a high value (step 424),and microprocessor 300 measures and stores the carbon dioxide sensorreading at this high level and the actual carbon dioxide level (e.g.,from operator input) (step 426). While the carbon dioxide level is stillat this high value, microprocessor 300 measures the incubatortemperature using temperature sensor 314 and the appropriatelinearization lookup table 334, and measures the relative humidity usinghumidity sensor 316 and the conversion lookup table 334, and finallycomputes the grains (humidity) within the incubator at the time of thehigh carbon dioxide reading (step 428). At this point, microprocessor300 is ready to begin carbon dioxide readings and control of carbondioxide levels in the incubator to a set point.

Each reading obtained from carbon dioxide sensor 312 is compensated by asequence of steps beginning with step 430 and the following steps.Initially, microprocessor 300 reads the carbon dioxide sensor and thenuses a linear interpolation procedure to compute an uncompensated carbondioxide reading. This linear interpolation procedure determines therelationship between the carbon dioxide sensor reading and the low andhigh readings obtained in steps 422 and 426. Then, the relationshipbetween the current reading and the low and high readings is used tointerpolate a carbon dioxide level based upon the actual carbon dioxidelevels at the low and high readings. The specific computation for thisstep is identified in FIG. 13A, step 430.

After thus determining an uncompensated carbon dioxide reading,microprocessor 300 proceeds to compensate this reading for sensitivitiesto humidity variation. First (step 432), microprocessor 300 measures thetemperature in the incubator using temperature sensor 314 andlinearization table 334. Next, microprocessor 300 determines therelative humidity in the incubator using humidity sensor 316 and theconversion table 334. Finally the relative humidity reading is convertedto grains using a table for converting a temperature and relativehumidity value into a grains value, which table is stored in memory 330as one of the tables 336.

After this initial preparation, microprocessor 300 proceeds to step 434in which it determines the change in grains in the incubator bysubtracting the grains at the current time from the grains measured instep 428 when the high carbon dioxide sensor reading was taken. Thischange in grains in the incubator is used to compensate for variationsin the carbon dioxide sensor reading. Specifically, in step 436, thecurrent temperature reading is used to determine the change in grains inthe incubator that causes a 1% apparent change in the carbon dioxideconcentration. This value is multiplied by the actual change in grainsfrom the time of the high carbon dioxide reading to obtain a resultingapparent change in the carbon dioxide reading which is attributable tohumidity change (step 436). This apparent change in carbon dioxidereading is then added to the uncompensated carbon dioxide reading toobtain a compensated carbon dioxide reading (step 438). The compensatedcarbon dioxide reading is then compared to the user-defined set point tocontrol the gas flow into the incubator (step 440).

The steps discussed above can be diagrammed as shown in FIG. 13B.Specifically, the temperature sensor reading at the high carbon dioxidecalibration 441 and the relative humidity sensor reading at the highcarbon dioxide calibration 442 are processed through a temperaturelinearization table 334 and relative humidity conversion table 334 toobtain the actual temperature and relative humidity at the high carbondioxide calibration. The temperature is then passed through a table 336to obtain the number of grains which correspond to 100% relativehumidity at the temperature of high carbon dioxide calibration. Next,the relative humidity is divided by one hundred and multiplied by thisvalue to produce a value 444 corresponding to the grains at the time ofthe high carbon dioxide calibration.

Later, when a linearized carbon dioxide sensor reading (445) isobtained, the difference between this sensor reading and the linearizedsensor reading at low carbon dioxide calibration (446) is computed andthis difference is compared to the difference between the linearizedcarbon dioxide sensor reading at low calibration (446) and thelinearized carbon dioxide sensor reading (448) at high calibration. Thequotient of these two differences (450) is then multiplied by thedifference between the actual carbon dioxide level at the highcalibration (452) and the actual carbon dioxide level at low calibration(454). The result (456) is a linearized, uncompensated carbon dioxidereading.

The current relative humidity sensor reading (460) and currenttemperature sensor reading (462) are processed through a relativehumidity conversion table 334 and temperature linearization table 334 toproduce current relative humidity and current temperature readings (461,463). The current temperature reading 463 is then processed through thetemperature-relative humidity table 336 to obtain the number (464) ofgrains that correspond to 100% relative humidity at the currenttemperature. This value is combined with the current relative humidity(461) to produce a number (465) indicative of the current grains in theincubator. The difference between this number and the grains at the timeof high carbon dioxide calibration (444) indicates the change in grains(470) since high carbon dioxide calibration.

The current temperature is passed through a second calibration table 336to create a value (472) indicating the change in grains that causes a 1%apparent change in carbon dioxide concentration at the currenttemperature. This value is combined with the change in grains (470) toproduce the apparent change in carbon dioxide due to humidity change(474). This value is added to the uncompensated carbon dioxide sensorreading (456) to produce a humidity compensated carbon dioxide reading(476).

In accordance with the foregoing, variations in apparent carbon dioxidereadings caused by humidity changes are compensated through the use ofthe various lookup tables identified.

Lookup table 336 identifying the number of grains that correspond to100% relative humidity at each given temperature can be generated fromtables in any handbook of chemistry and physics indicating theoreticalrelationships between relative humidity, temperature and grains. Thesecond table 336 for converting a temperature into the change in grainsthat causes a 1% apparent change in carbon dioxide at that temperature,may be generated by empirical measurements created by varying thehumidity, carbon dioxide and temperature levels in a incubator, andmeasuring carbon dioxide sensor outputs to determine the amount ofcarbon dioxide sensor variation caused by changes in grains at varioustemperatures.

A similar procedure is used to compensate the carbon dioxide readingsfor variations in oxygen content within the incubator. Referring to FIG.14A, this procedure also begins with the calibration of the carbondioxide sensor. Specifically, after step 424 when the incubator carbondioxide has been increased to a high level, the oxygen level in theincubator is measured (step 480) and stored for later use incalibration.

Later, the carbon dioxide sensor output is calibrated for oxygenvariation by measuring the oxygen level in the incubator and thetemperature (and linearizing the temperature reading using a table 334)(step 482). Next, the change in oxygen is determined by subtracting theoxygen level at the high carbon dioxide calibration from the currentoxygen reading (step 484). Then, the current temperature is used todetermine the change in oxygen that would cause a 1% apparent change inthe carbon dioxide reading. This is multiplied by the actual change inoxygen to obtain the resulting apparent change in carbon dioxide (step486). Finally, this apparent change in carbon dioxide is added to theuncompensated carbon dioxide reading to produce a compensated carbondioxide reading (step 488). Subsequently, as discussed above, thecompensated carbon dioxide reading is compared to the set point, andused to control the gas flow into the incubator as appropriate (step440).

The procedure described above may also be illustrated graphically asseen in FIG. 14B. Specifically, the oxygen reading (490) obtained duringhigh carbon dioxide calibration, is subtracted from the current oxygenreading (492) to obtain a value (493) indicative of the change in oxygensince high carbon dioxide calibration. Next, the linearized currenttemperature reading (463) is passed through a table 336 to obtain avalue (494) indicating the change in oxygen that would cause a 1%apparent change in a carbon dioxide reading at the current temperature.The change in oxygen (493) is then divided by the amount of oxygenchange that would cause a 1% apparent change in carbon dioxide (494) toobtain the apparent change in carbon dioxide which is due to changes inthe oxygen level (495). This value (495) is then added to the carbondioxide reading to obtain a compensated carbon dioxide reading.

The above discussed table 336 indicating the change in oxygen that wouldcause a 1% apparent change in a carbon dioxide reading at varioustemperatures, may be generated empirically by applying varioustemperatures oxygen concentrations and carbon dioxide concentrations toa thermal conductivity carbon dioxide sensor, to determine for eachtemperature, and oxygen concentration change, the amount of apparentcarbon dioxide change produced. The resulting values stored in the table336 can then be used in the manner discussed to produce oxygen levelcompensation of the carbon dioxide sensor.

Referring now to FIG. 15A, in one embodiment of the invention, aninfrared carbon dioxide sensor is used to sense the carbon dioxidelevels in the incubator, rather than a thermal conductivity sensor. Asnoted above, an infrared sensor is more expensive than a thermalconductivity sensor; however, an infrared sensor is not subject tosubstantial variation as a result of oxygen or humidity.

The infrared sensor is calibrated using zero end span potentiometerswhich are connected to the infrared sensor at terminals on the sensor.As seen in FIG. 15C, the infrared sensor 500 produces an output signalon line 315, and includes terminals 501 for connection to apotentiometer for adjusting the zero of the sensor output signal on line315, and terminals 502 for connection to a potentiometer for adjustingthe gain or span of the output signal produced on line 315. Sensor 500also produces a 6 volt source signal on terminal 503 and is responsiveto a reference signal on terminal 504 to drive the infrared light sourceused by the sensor.

In accordance with principles of the present invention, digitalpotentiometers 506 and 508 are connected to terminals 501 and 502,respectively, to control the zero and span of the output on line 315.Furthermore, the 6 volt source on line 503 produced by the sensor, and a5 volt source 510, are alternately connected to input terminal 504 aspart of the calibration procedure, which is discussed below. The voltageon line 504 and the sensor output signal on line 315 are digitized bythe AID converter 310 and delivered to microprocessor 300.

Microprocessor 300 controls digital potentiometers 506 and 508 and alsoreferences stored curves found in a table 336, and a linearization table334, in memory 330 to calibrate and linearize the infrared sensor outputsignal on line 315.

The 5 or 6 volt reference signal is alternately applied to input line504 by attaching a jumper 512 between line 504 and either line 503 orline 505. When the jumper 512 is connected between line 504 and line505, the 5 volt source 510 is applied to the input terminal 504,resulting in a low level light output from the infrared sensor'sinternal infrared light source. When jumper 512 is connected betweenline 504 and line 503, the sensor's internal 6 volt source is applied tothe input terminal 504, resulting in a normal, high level light outputfrom the infrared sensor's internal infrared light source. As drawn inFIG. 15C, jumper 512 is positioned to connect the 5 volt source 510 tothe sensor's input line 504.

Referring now to FIG. 15A, to calibrate the infrared sensor 500, jumper512 is initially positioned between lines 503 and 504, to connect thenormal 6 volt source to terminal 504. The calibration procedure followedby microprocessor 300 begins at step 520 by waiting until the normalsource voltage is sensed on terminal 504. A/D converter is used todetermine the approximate voltage on line 504 and processing continuesto step 522 only when an approximately 6 volt voltage is detected online 504.

Once the normal source voltage is applied to the infrared sensor 500, instep 522 microprocessor 300 controls the incubator to expose theinfrared sensor to room air, which is 0.033% carbon dioxide, so that theinfrared sensor is exposed to a known, predetermined carbon dioxideconcentration. Once the sensor has been sufficiently exposed to roomair, microprocessor 300 adjusts the digital potentiometer 506, to changethe infrared sensor output until it has an output value of zero (step524). This establishes the zero setting for the infrared sensor 500 suchthat a room air carbon dioxide concentration produces a zero output online 315.

Next, in step 526, microprocessor 300 waits until a reduced sourcevoltage of approximately 5 volts is applied to input line 504 of sensor500. To do this, the user must move jumper 512 to the position shown inFIG. 15C. Only when microprocessor 300 detects a roughly 5 volt voltageon line 504, it will proceed to step 528.

In step 528, microprocessor 300 retrieves from memory 330 a stored curvefrom a table 336. As illustrated in FIG. 15B, this table stores a numberof curves 529. Each of these curves indicates the sensor output as afunction of infrared source voltage when the sensor is exposed toambient air having a carbon dioxide concentration of 0.033%. In step528, microprocessor 300 locates the curve 529 which has zero sensoroutput voltage at the source voltage detected on line 504 in step 520.This curve describes the state of operation of the infrared sensor 500.

In step 530, microprocessor 300 uses the retrieved curve to predict thesensor output which should be produced by the approximately 5 voltsource voltage produced by source 510, and which is currently beingapplied to line 504. In step 532, microprocessor 300 adjustspotentiometer 508 to change the infrared sensor output voltage on line315 until the sensor output voltage is equal to the predicted voltage.By doing so, microprocessor 300 adjusts the gain in the infrared sensorto normalize the behavior of the infrared sensor to the selected curve529.

After thus setting the gain (span) of the infrared sensor,microprocessor 300 again waits in step 534 until the normal sourcevoltage is reapplied to source voltage terminal 504 of infrared sensor500. During this waiting time, the operator must move the jumper 512from the position shown in FIG. 15C to the position shown in dottedoutline in FIG. 15C. Once the jumper has been moved, and microprocessor300 detects a normal source voltage applied to terminal 504,microprocessor 300 moves to step 536, and (if necessary) readjustspotentiometer 506 to change the infrared sensor output to produce a zerooutput. This second readjustment of potentiometer 506 eliminatespossible zeroing errors produced during adjustments of potentiometer508.

After this calibration procedure has been completed, microprocessor 300has adjusted the infrared sensor into calibration, such that the sensoroutput 315 can subsequently be used to measure carbon dioxide, byretrieving the sensor output voltage 315 via A/D converter 310 andlinearizing the output voltage using a linearization table 334 stored inmemory 330, as discussed above.

It will be noted that the above-described calibration procedure does notrequire altering the carbon dioxide concentration in the incubator, butrather merely involves applying room air to the carbon dioxide sensor.Known carbon dioxide sensor calibration procedures require exposing thesensor to different carbon dioxide concentrations, which is not onlytime consuming but also consumes resources of carbon dioxide gas. Theabove-described procedure is thus highly advantageous as compared tothese known procedures.

While preferred embodiments of the various aspects of the invention havebeen described in detail, those of ordinary skill will recognizemodifications thereof still falling within the spirit and scope of theinventive concepts.

What is claimed is:
 1. A laboratory incubator comprising: a cabinetincluding a chamber having an interior incubating space partiallydefined by an upper panel and a side panel, said upper panel includingan aperture; an upper plenum positioned behind said upper panel; a sideplenum connected for fluid communication with said upper plenum and saidinterior incubating space such that an air flow path is formed from saidinterior incubating space, through said upper and side plenums and backinto said interior incubating space; a door connected with the cabinetto allow access to said interior incubator space and said upper plenum;a heater connected in thermal communication with said chamber, includingsaid interior incubating space and said side plenum, for supplying heatto said interior incubating space, said upper plenum and said sideplenum; an air moving device mounted in said upper plenum forrecirculating air through said air flow path; and a filter removablymounted within said interior incubating space adjacent said aperture,said filter being accessible through said door and operating to filterair as the air circulates in said air flow path.
 2. The laboratoryincubator of claim 1, wherein said filter further comprises a HEPAfilter.
 3. The laboratory incubator of claim 2, wherein said HEPA filteris circular and is positioned directly below said air moving device. 4.The laboratory incubator of claim 3, wherein said HEPA filter issubstantially smaller in size than said upper panel.
 5. The incubator ofclaim 4, wherein said air moving device includes an inlet portion influid communication with a central portion of said circular HEPA filter.6. The laboratory incubator of claim 5, wherein said air moving devicecauses the air in said air flow path to move from a lower portion ofsaid interior incubator space toward the upper panel, through said HEPAfilter, across said upper plenum, down said side plenum and back intosaid interior incubating space.
 7. The laboratory incubator of claim 6,wherein said air moving device is a blower, and said blower isaccessible through said door and said interior incubating space.
 8. Alaboratory incubator comprising: a cabinet including a chamber having aninterior incubating space partially defined by an upper panel and a sidepanel, said upper panel including an aperture; an upper plenumpositioned behind said upper panel; a side plenum connected for fluidcommunication with said upper plenum and said interior incubating spacesuch that an air flow path is formed from said interior incubatingspace, through said upper and side plenums and back into said interiorincubating space; a door connected with the cabinet to allow access tosaid interior incubator space and said upper plenum; a heater connectedin thermal communication with said chamber, including said interiorincubating space and said side plenum, for supplying heat to saidinterior incubating space, said upper plenum and said side plenum; anair moving device mounted in said upper plenum for recirculating airthrough said air flow path; and a circular filter removably mountedwithin said interior incubating space below said upper panel andadjacent said aperture, said circular filter being accessible throughsaid door and operating to filter air as the air circulates in said airflow path, said filter further including a central portion aligned withsaid aperture and in fluid communication with said air moving device. 9.The laboratory incubator of claim 8, wherein said circular filterfurther comprises a HEPA filter.
 10. The laboratory incubator of claim9, wherein said HEPA filter is substantially smaller in size than saidupper panel.
 11. The laboratory incubator of claim 8, wherein said airmoving device causes the air in said air flow path to move from a lowerportion of said interior incubator space toward the upper panel, throughsaid circular filter, across said upper plenum, down said side plenumand back into said interior incubating space.
 12. The laboratoryincubator of claim 8, wherein said air moving device is a blower, andsaid blower is accessible through said door and said interior incubatingspace.
 13. A laboratory incubator comprising: a cabinet including achamber having an interior incubating space partially defined by anupper panel and a side panel, said upper panel including an aperture; anupper plenum positioned behind said upper panel; a side plenum connectedfor fluid communication with said upper plenum and said interiorincubating space such that an air flow path is formed from said interiorincubating space, through said upper and side plenums and back into saidinterior incubating space; a door connected with the cabinet to allowaccess to said interior incubator space and said upper plenum; a heaterconnected in thermal communication with said chamber, including saidinterior incubating space and said side plenum, for supplying heat tosaid interior incubating space, said upper plenum and said side plenum;an air moving device mounted in said upper plenum for recirculating airthrough said air flow path; and a circular filter removably mountedwithin said interior incubating space below said upper panel andadjacent said aperture, said circular filter formed by a sandwichconstruction of upper and lower face portions and annularly configuredfilter material disposed between said upper and lower face portions,said circular filter being accessible through said door and operating tofilter air as the air circulates in said air flow path, said upper faceportion and said filter material further including respective centralportions aligned with said aperture and in fluid communication with saidair moving device.
 14. The laboratory incubator of claim 13, whereinsaid circular filter further comprises a HEPA filter.
 15. The laboratoryincubator of claim 14, wherein said HEPA filter is substantially smallerin size than said upper panel.
 16. The laboratory incubator of claim 13,wherein said air moving device causes the air in said air flow path tomove from a lower portion of said interior incubator space toward theupper panel, through said circular filter, across said upper plenum,down said side plenum and back into said interior incubating space. 17.The laboratory incubator of claim 13, wherein said air moving device isa blower, and said blower is accessible through said door and saidinterior incubating space.